initial version of performance profiling document (microsoft#869)

* initial version of performance profiling document

* Update PerformanceProfilingWindows.md

Author: dryganets

docs/PerformanceProfilingWindows.md

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+# Universal Windows Platform performance analysis
+
+## Tools
+1. [Windows Performance Analyzer](https://msdn.microsoft.com/en-us/windows/desktop/hh448170.aspx) - the only usable tool so far
+2. Standard profiler - doesn't give much information, but still could be useful
+3. [DotTrace](https://www.jetbrains.com/profiler/) profiler from JetBrains, gives the same level of info as Windows Performance Analyzer
+4. [Intel vTune](https://software.intel.com/en-us/vtune) - Does not support UWP or WPF-specific profiling, but gives low-level CPU and GPU breakdowns.
+
+## Windows Performance Analyzer
+You should have all the tools downloaded as part of UWP sdk, but in case you don't - download the latest version of the Windows Performance Tools Kit, and install it on your machine. (http://www.microsoft.com/whdc/system/sysperf/perftools.mspx : Windows Performance Tools Kit, v.4.1.1 (QFE))
+To utilize this powerful tool you need to turn on application Tracing first.
+
+To do so you need to create an instance of Windows.Foundation.Diagnostic.FileLoggingSession class and add Tracer.Instance channel to this session:
+
+```
+       static Tracer()
+        {
+            loggingSession = new FileLoggingSession("performance");
+            loggingSession.AddLoggingChannel(Instance);
+        }
+
+```
+
+This will enable application level tracing for react-native code.
+You could start and stop application level profiling dynamically by adding and removing the channel.
+
+I think it would make sence to hide tracing under define and add context menu for profiling features later.
+
+You would see the logs in following folder:
+~\AppData\Local\Application Data\Packages\%PACKAGE_UID%\LocalState\Logs\
+log name would be: Log-performance-*.etl
+it rotates every 270Kb or so.
+
+I would recommend you to record additional system level information using xperf tool.
+TODO: find out how to scope tool results to specific process (ideally add to the same file session)
+
+To start event recording run:
+```
+xperf start -on DiagEasy 
+```
+To stop recording run:
+```
+xperf stop -d filename.etl
+```
+
+Windows Performance Analyzer works with single file so you need to merge all your files:
+
+```
+xperf merge *.etl merge.etl
+```
+
+After that you could open the resulting merge.etl file in WPA tool and see what happens inside of react code.
+
+
+## More info
+1. https://blogs.msdn.microsoft.com/ntdebugging/2008/04/03/windows-performance-toolkit-xperf/
+2. https://msdn.microsoft.com/en-us/library/windows/hardware/hh448170.aspx
+
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